1. Field of the Invention
The present invention concerns the structure of electron sources and, by way of application, tubes for the emission of very high frequency and very high power electromagnetic waves.
2. Description of the Prior Art
In many fields, it is necessary to be able to produce electron beams, the energy of which is used to produce secondary phenomena, for example, the emission of electromagnetic waves. This is the case with microwave tubes which are commonly a part of the structure of electronic control systems, for example for radars.
At present, the source which produces an electron beam in tubes of this kind consists of a thermionic cathode heated notably by Joule effect. This technique enables the fairly easy production of the electrons with a desired density but it, nevertheless, has a drawback when it is necessary to modulate this electron density. For, to obtain this modulation, it becomes necessary to associate well known, ancillary electrodes with these cathodes which, to put it very schematically, consists of gates to which variable electrical potentials are applied. The performance performance characteristics of these sources are quickly limited once the frequency goes beyond 1 MHz, owing to the transit time.
Approaches have been proposed to try and overcome the above-mentioned drawbacks. One of them is the lasertron which uses the photoelectric effect. An incident laser pulse, modulated at the desired frequency, excites the current of electrons by photo-emission from a cathode having sound photo-emissive properties. This approach requires a laser modulated at very high frequency and a cathode having very stable photo-emissive properties. These conditions make a source of this type hardly usable in industry, because of its complexity and its high cost price.
In one approach, it has also been proposed to use a solid based on semiconductor materials, essentially formed by two layers of materials such as gallium arsenide with two different dopings between which an electrical voltage is applied. One of the two layers enables the release of a certain quantity of electrons which are then subjected to the electrical field created by the voltage. They are therefore accelerated and then fall on the second layer which enables their number to be greatly increased. When they have acquired sufficient energy, they are ejected from the solid, especially by the second layer, the extraction role of which is accentuated by the addition of a further layer on its external face, for example a layer of cesium or cesium oxide. This additional layer is aimed at lowering the electron affinity of the semiconductor, and all the electrons coming to this conduction zone can thus have enough energy to get extracted from the second layer, and hence from the solid.
However, this latter approach enables only an extraction of electrons that is continuously or modulated at low frequency, and it is still impossible to achieve quick modulation of the quantity of electrons ejected so as to obtain, notably in the case of electron sources for microwave tubes, successive electron packets enabling the creation of modulated electromagnetic waves.
It is an object of the present invention to overcome the above-mentioned drawbacks and to propose an electron source with a simple structure which can be used in many fields, such that its cost price makes it an industrial product for large-scale use and, above all, a structure which can deliver a quantity of electrons that can be very easily modulated.